I can tell you a bit about the situation in Scotland, which I think will be similar to that elsewhere. As people have already said, a trust is the best thing to do, but it has to be a discretionary trust (ie, the trustees have the discretion to refuse or grant a request).
It's better to have a "live" trust, I believe, although I'm not sure why. This means that the trust comes into operation before you die. Obviously, this can only happen if you have some money to put into it. Friends and relatives can also add to the trust.
If you want to leave your property to your child so that he or she can continue to live in it, it can also be left to the trust, but you'd have to make sure there was enough money in the trust to maintain the property.
Money held in trust is not taken into account when calculating benefits, BTW.
No-one can be power of attorny for your DD if she does not have capapcity to give it, but you can (in Scotland, anyway) apply to a court for someone to be a welfare guardian (and/ or a financial guardian).
I think Mencap has a trust service, where they would administer the trust, with your appointed trustees, for a small annual fee.
I have a brochure about wills and one about trusts that I can send you if you CAT me.
It's a minefield, and a really difficult thing to think about. I hope you get it sorted out.